This invention relates to sintered silicon nitride products having high strength at high temperatures and excellent oxidation resistance.
Sintered silicon nitride products have excellent properties such as mechanical strength, heat and corrosion resistance, etc., and they are used as high temperature structural materials, such as for parts of automotive engines, gas turbines, etc., which require heat resistance and oxidation resistance.
Silicon nitride has a poor sinterability because of its strong covalent bond. Therefore, sintered silicon nitride products have been fabricated by using sintering aids such as magnesium oxide (MgO), aluminum oxide (Al.sub.2 O.sub.3), zirconium dioxide (ZrO.sub.2), oxide of rare earth elements, etc..
However, when the metallic oxides such as MgO, Al.sub.2 O.sub.3, and/or ZrO.sub.2 is used as sintering aids, low-melting point glass phase is formed as a grain boundary phase, resulting in degradation of strength at high temperatures and inferior oxidation resistance.
When the oxides of any rare earth elements are used as sintering aid, hot-pressing process is required to obtain dense sintered products, because these sintering aids are less effective in densification than other sintering aids.
The nature and functioning of sintering aids to be added in fabricating sintered products have been extensively studied, and a sintering aid excellent in sinterability has been developed.